It has been proposed to prepare polyesters from lower dialkyl esters of terephthalic and isophthalic acid, ethylene glycol, and a higher polyfunctional alcohol such as glycerine, for use as an electrical insulating material (see U.S. Pat. No. 2,926,296 issued May 10, 1960).
It has also been proposed to prepare polyester-polyimides for use as a wire enamel (U.S. Pat. No. 3,426,098, issued Feb. 4, 1969). According to the latter patent the polyester/polyimide is prepared from (1) tris (2-hydroxy-ethyl) isocyanurate, (2) terephthalic or isophthalic acid or the lower alkyl esters thereof, (3) an aromatic diamine, e.g., oxydianiline, or methylene dianiline, and (4) an aromatic carboxylic acid anhydride containing at least one additional carboxyl group, e.g., trimellitic anhydride or pyromellitic anhydride. See also U.S. Pat. No. 4,107,355.
While it is well known that polymers useful for wire enamels can be made by reacting aliphatic diols and polyols with aromatic polycarboxylic acids or their alkyl esters, there has been lacking up to the present time, a practical synthesis for incorporating an aromatic nucleus as the backbone of the polyol constituent. Attempts to substitute an aromatic polyol, such as Bisphenol A, for the aliphatic diol in the synthesis mentioned above have met with little or no success.
We have now discovered that the desirable properties possessed by an aromatic polyol, such as Bisphenol A, can be incorporated into a polymer backbone, by reacting an aromatic ester of an aromatic dicarboxylic acid directly with the desired aromatic polyol, such as Bisphenol A. In such reaction, the phenol forming the aromatic ester is smoothly eliminated, as a by-product and the desired reaction is carried out easily to completion.